In traction sheave elevators, the elevator car and counterweight are suspended on round steel ropes. Normally, the same ropes act both as suspension ropes, whose function is to support the elevator car and counterweight, and as hoisting ropes serving to move the elevator car and counterweight. Therefore, the ropes must be designed to carry the entire load, even if, when a counterweight is used, the force needed to move the elevator is very small—in an extreme case nearly zero when the counterweight and the elevator car with the car load are equal in weight.
In prior art, there are also solutions having separate suspension ropes and hoisting ropes. Such an elevator is presented e.g. in U.S. Pat. No. 5,398,781. In the elevator described in this specification, the suspension rope is attached to the top part of the elevator car and passed via diverting pulleys to a lever element on the counterweight. The hoisting rope is attached either to the top or bottom part of the elevator car and, like the suspension rope, passed via diverting pulleys and the traction sheave of the hoisting machine to a lever element on the counterweight. To compensate for rope elongation, the elevator described in this specification comprises a lever element fitted in conjunction with the counterweight and acting as a tensioning device. This patent focuses especially on the tensioning of the hoisting rope and contains no mention of any details of the suspension ropes or hoisting ropes. Neither does it describe any advantages that could be achieved by separate implementation of hoisting ropes and suspension ropes.
The hoisting ropes generally used are steel cables, whose friction coefficient is, however, so low that it has to be increased e.g. by using traction sheaves with different types of grooves or by increasing the angle of contact or angle of rotation of the rope around the traction sheave. In addition, a hoisting rope made of steel functions as a kind of sound bridge between the hoisting motor drive and the elevator car, transmitting noise from the hoisting machinery to the elevator car and thus impairing passenger comfort.
A further drawback with prior-art solutions using steel hoisting ropes is that the bending radius of the rope is relatively large, which means that the traction sheave and diverting pulleys must have a large diameter. Another drawback with steel rope is that the weight of the rope imposes a limit on the hoisting height of elevators. Moreover, steel ropes liable to corrosion, so they require regular maintenance.
Specification EP 672 781 A1 presents a round elevator suspension rope made of synthetic fibers. Topmost on the outside it has a sheath layer surrounding the outermost strand layer. The sheath layer is made of plastic, e.g. polyurethane. The strands are formed from aramid fibers. Each strand is treated with an impregnating agent to protect the fibers. Placed between the outermost and the inner strand layers is an intermediate sheath to reduce friction. To achieve a nearly circular strand layer and to increase the volumetric efficiency, the gaps are filled with backfill strands. The function of the top-most sheath layer is to ensure a coefficient of friction of desired magnitude on the traction sheave and to protect the strands against mechanical and chemical damage and UV radiation. Thus, the load is supported exclusively by the strands. As compared with corresponding steel rope, a rope formed from aramid fibers has a substantially larger load bearing capacity and a specific weight equal to only a fifth or a sixth of the specific weight of corresponding steel rope.
A drawback with these prior-art solutions, in which a round elevator rope formed e.g. from synthetic fibers, is that the rope has a relatively large bending radius, requiring the use of large-diameter traction sheaves and diverting pulleys. Further, there occurs a fair deal of sliding of the strands and fibers in relation to each other. Moreover, the ratio of volume to area is high, which means that frictional heat will not be effectively removed from the rope and the rope temperature is therefore liable to rise unduly.